There is an infinite amount of work to do.

Because you can look into the future, you can see that things could always be better with a bit more work. Maybe you’ve prepared for a flood, but have you also prepared for a famine, or a plague? There’s no limit to the number of things that could wipe you out if you are unprepared, so there’s no limit to the preparation that you will have to do.

People who are pursuing FIRE can find it difficult to stop working even once they hit their number. I suspect that the sort of forward-looking mindset that gets you into FIRE in the first place is the same sort of mindset that sees that every additional bit of money is an additional bit of security.